Introduction to Transient Stability Starrett Mini-Lecture #1

Steady-state = stable equilibrium

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things are not changing concerned with whether the system variables are within the correct limits

Transient Stability

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"Transient" means changing The state of the system is changing We are concerned with the transition from one equilibrium to another The change is a result of a "large" disturbance

Primary Questions

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1. Does the system reach a new steady state that is acceptable?

2. Do the variables of the system remain within safe limits as the system moves from one state to the next?

Generally concerned with the synchronism of synchronous machines in the system

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Instability => at least one rotor angle becomes unbounded with respect to the rest of the system Also referred to as "going out of step" or "slipping a pole"

May also be concerned with other limits on other system variables

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Transient Voltage Dips Short-term current & power limits

Time Frame

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Typical time frame of concern

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1 - 30 seconds Model system components that are "active" in this time scale Faster changes -> assume instantaneous Slower changes -> assume constants

Primary components to be modeled

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Synchronous generators

Traditional control options

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Generation based control

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exciters, speed governors, voltage regulators, power system stabilizers

Traditional Transmission Control Devices

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Slow changes modeled as a constant value

FACTS Devices

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May respond in the 1-30 second time frame modeled as active devices

May be used to help control transient stability problems

Kundur's classification of methods for improving transient stability

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Minimization of disturbance severity and duration Increase in forces restoring synchronism Reduction of accelerating torque by reducing input mechanical power Reduction of accelerating torque by applying artificial load

Commonly used methods of improving transient stability

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High-speed fault clearing, reduction of transmission system impedance, shunt compensation, dynamic braking, reactor switching, independent and single-pole switching, fast-valving of steam systems, generator tripping, controlled separation, high-speed excitation systems, discontinuous excitation control, and control of HVDC links

FACTS devices = Exciting control opportunities!

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Deregulation & separation of transmission & generation functions of a utility FACTS devices can help to control transient problems from the transmission system

3 Minute In-Class Activity

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1. Pick a partner 2. Person wearing the most blue = scribe Other person = speaker 3. Write a one-sentence definition of "TRANSIENT STABILITY” 4. Share with the class